This invention relates to the digestion of cellulosic material such as wood chips in a batch-type process. The invention more particularly concerns a process for digesting cellulosic material featuring an efficient and practical manner of recovering heat used in the process.
In a conventional batch process for digesting wood chips, a digester is filled with chips and the digester is then charged with cooking chemical, which in a soda process comprises essentally a solution of sodium hydroxide, and in a kraft process comprises such a solution with the further inclusion of a sulphur compound. The digester is then sealed, and with steam the temperature of the digester is brought up to cooking temperature. At the conclusion of the cook a blow valve in the digester is opened and the contents of the digester discharged into a blow tank. Much of the heat energy acquired by the contents of the digester during the processing of the pulp exits through the blow tank with exhaust vapors. To recover such energy attempts have been made to pass such vapors through various forms of heat recovery systems. These recovery systems, however, have not been truly efficient. To conserve energy costs, some pulp manufacturers have chosen to install continuous digestion processes. Such a process ordinarily is characterized by a more efficient utilization of heat than is achieved with a conventional batch process. However, the cost of the equipment needed in a continuous process is substantially greater than the cost of the equipment required in a batch-type process.
The prior patent art of which I am aware illustrates various attempts of others to recover heat in spent liquor produced in a pulping process. An example of such prior art is U.S. Pat. 1,697,032 disclosing a sulphite process for cooking pulp. In the process described in this patent, hot liquor recovered in a blow pit is caused to flow down through a digester together with wood chips, thereby to obtain greater packing of the chips, and also some heating of the chips. However, with this process, as in the case of conventional batch-type soda and kraft processes described above, in discharging the contents of the digester into the blow pit, considerable energy exits from the blow pit with exhaust vapors.
U.S. Pat. No. 2,195,378 discloses a process for chemically producing pulp, wherein white liquor is heated within an accumulator by a heating coil heated by pumping spent liquor through the coil. There are practical problems involved in removing spent liquor from a cooked mixture in a digester using a pump as disclosed. Furthermore, it is difficult to recover heat energy in a practical manner from spent liquor having the temperature that such has when leaving the heating coil described in the patent.
Generally, therefore, it is an object of the invention to provide a process for the batch-type digestion of cellulosic material which makes possible the efficient recovery of heat energy in a practical manner.
With steam being the usual source of energy for operating a digestion process, a corollary of the above is to provide a batch-type digestion process having reduced steam requirements.
A further object of the invention is to provide a method for digesting cellulosic material, wherein heat is recovered from spent liquor residing in the digester after the final cook in a manner promoting efficient recovery even though relatively low temperature levels are involved.
As contemplated by the invention, spent liquor in the digester after cooking of the cellulosic material is displaced from the digester by admitting a displacing liquid which expels the spent liquor while replacing it in the digester. Pressure in the digester is maintained during this displacement to prevent flashing of the liquor. Spent liquor recovered in this manner is then utilized to supply heat to a subsequent digester charge.
Further contemplated in a specific embodiment of the invention is the collection of liquids displaced from a digester at two temperature levels. Liquid at a lower temperature level may be utilized in the initial preheating of a charge of cellulosic material within a digester by immersing the cellulosic material level may be utilized in further heating the cellulosic material, through displacement of the lower temperature liquid within the digester with the higher temperature liquid.
Other features and advantages of the invention include a more efficient use of cooking chemical in the digestion process and reduced use of cooking chemical; higher yields and improved quality of pulp by reason of greater selectivity in the delignification reaction which occurs; the capability of using the digester as an instrumentality performing a washing operation in the process; and control of scaling in heat exchanger equipment. Furthermore, existing facilities for carrying out batch digestion processees are readily converted to perform the method of the invention.